Graft polymers have recently been recognized as being useful as highly functional high polymeric materials and have come into wide use in industry.
Graft polymers have conventionally been produced by polymerizing a graft monomer by solution polymerization, bulk polymerization or emulsion polymerization in the presence of a high-molecular weight compound as a main polymer which comprises a monomer different from the graft monomer by using a polymerization initiator, such as a radical polymerization initiator. The graft polymers obtained by these processes have low purity due to the presence of a large proportion of a non-grafted polymer.
In recent years, a process comprising copolymerizing a high-molecular weight monomer having a polymerizable group at the terminal thereof, called a macromonomer, with other copolymerizable monomers (hereinafter referred to as a macromonomer process) has been developed and has attracted attention as a process capable of providing graft polymers at a high grafting efficiency.
The production of graft polymers according to the macromonomer process has generally been carried out by solution polymerization using a solution of a macromonomer and a copolymerizable monomer in an organic solvent by or suspension polymerization using an aqueous suspension of the monomers. However, solution polymerization does not meet present demands for environmental conservation and preservation of resources. On the other hand, since graft polymers obtained by suspension polymerization undergo phase separation into a liquid phase and a solid phase, it is difficult to handle them as an aqueous dispersion in a stable manner. It has been keenly desired, therefore, to develop a graft polymer of the aqueous emulsion type comprising finely divided particles.
There have been several proposals for production of graft polymers by aqueous emulsion polymerization according to a macromonomer process. For example, emulsion copolymerization of a polyisobutylene type macromonomer having a p-vinylphenyl group at one terminal thereof and styrene is reported by Joseph P. Kennedy et al. in Polymer Bulletin, No. 13, pp. 441-446 (1985). According to the report, the above-described macromonomer is dissolved in a styrene monomer, and the solution is emulsified in water with sodium nonylphenoxypolyethoxyethanol sulfate to prepare a finely divided aqueous emulsion having an average particle size of about 0.24 .mu.m. The resulting aqueous emulsion is then polymerized in the presence of oil-soluble azobisisobutyronitrile as a polymerization initiator to obtain a graft polymer in a high yield.
The inventors of the present invention attempted variations on the process of Kennedy et al., replacing the polyisobutylene type macromonomer with a polyacrylic ester type macromonomer, a styrene/acrylonitrile copolymer type macromonomer, and other macromonomoers suitable as coating resins, etc. As a result, they encountered a large amount of coagula during production and failed to obtain a satisfactorily emulsified dispersion.
JP-A-62-64814 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a process in which a macromonomer is dissolved in an organic solvent, e.g., toluene; an emulsifying agent is added thereto; a finely divided aqueous dispersion is prepared in the presence of water; a monomer copolymerizable with the macromonomer is added to the dispersion; and copolymerization is conducted in the presence of a radical polymerization initiator to prepare an aqueous emulsion of a graft polymer. According to this process, however, because the macromonomer forms micelles independently of the other copolymerizable monomer, polymerization among the macromonomer molecules or among the other copolymerizable monomer molecules proceeds predominantly over graft polymerization, resulting in poor production yield of the desired graft polymer.